1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine airfoil having a spar and shell construction with internal cooling.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
As one skilled in the gas turbine technology recognizes, the efficiency of the engine is enhanced by operating the turbine at a higher temperature and by increasing the turbine's pressure ratio. Another feature that contributes to the efficiency of the engine is the ability to cool the turbine with a lesser amount of cooling air. The problem that prevents the turbine from being operated at a higher temperature is the limitation of the structural integrity of the turbine component parts that are jeopardized in its high temperature, hostile environment. Scientists and engineers have attempted to combat the structural integrity problem by utilizing internal cooling and selecting high temperature resistant materials. The problem associated with internal cooling is twofold. One, the cooling air that is utilized for the cooling comes from the compressor that has already extended energy to pressurize the air and the spent air in the turbine cooling process in essence is a deficit in engine efficiency. The second problem is that the cooling is through cooling passages and holes that are in the turbine blade or vane which, obviously, adversely affects the blade or vane's structural prowess. Because of the tortuous path (a serpentine path through the blade or vane) that is presented to the cooling air, the pressure drop that is a consequence thereof requires higher supply pressure and more air flow to perform the cooling that would otherwise take a lesser amount of air given the path becomes friendlier to the cooling air. While there are materials that are available and can operate at a higher temperature that is heretofore been used, the problem is how to harness these materials so that they can be used efficaciously in the turbine environment.
Also well known by those skilled in this technology is that the engine's efficiency increases as the pressure ratio of the turbine increases and the weight of the turbine decreases. Needless to say, these parameters have limitations. Increasing the speed of the turbine also increases the airfoil loading and, of course, satisfactory operation of the turbine is to stay within given airfoil loadings. The airfoil loadings are governed by the cross sectional area of the turbine multiplied by the velocity of the tip of the turbine squared, or AN2. Obviously, the rotational speed of the turbine has a significant impact on the loadings.
One prior art reference, U.S. Pat. No. 7,080,971 B2 issued to Wilson et al. on Jul. 25, 2006 and entitled COOLED TURBINE SPAR AND SHELL BLADE CONSTRUCTION, discloses a spar/shell construction that affords the turbine engine designer the option of reducing the amount of cooling air that is required in any given engine design. And in addition, allowing the designer to fabricate the shell from exotic high temperature materials that heretofore could not be cast or forged to define the surface profile of the airfoil section. In other words, by virtue of this prior art turbine blade, the shell can be made from Niobium or Molybdenum or their alloys, where the shape is formed by a well known electric discharge process (EDM) or wire EDM process. In addition, because of the efficacious cooling scheme of this invention, the shell portion could be made from ceramics, or more conventional materials and still present an advantage to the designer because a lesser amount of cooling air would be required. The cooling arrangement in this Wilson et al. patent is a single impingement cooling circuit. The blade is formed of a single spar that provides a rigid support for the shell that forms the airfoil surface. The spar includes a single central cavity that forms the cooling air supply passage with impingement holes to direct impinging air against the inside surface of the spar wall.
It is an object of the present invention to provide for a turbine airfoil of the spar and shell construction in which the internal cooling circuit includes multiple impingement cooling of the airfoil.
It is another object of the present invention to provide for a turbine airfoil of the spar and shell construction in which the shell also forms part of the internal cooling channels.